The use of metallic powders suspended in a viscous carrier or applied in dry form directly to a substrate using a conventional silk screen process or another type of application process is known. Thereafter, the particles are cured to form electrical conductors such as disclosed in U.S. Pat. Nos. 4,278,702 and 5,014,420.
Among the shortcomings of prior art of which we are aware, such as embodied in these prior art patents, is the fact that metallic particles of substantially uniform size are used and, as a result of experimentation, we have discovered that considerable shrinkage in the deposited traces is experienced when the particles are melted, which could lead to unwanted pattern distortion or breaking up of the conductive traces into discontinuous balls. As best understood, it is theorized that distortion occurs since the use of powders of substantially uniform size results in voids between the individual particles such that, upon melting, the melted particles flow to fill these voids, causing shrinkage and separation of the particles forming the conductor line into the discrete balls which are spaced from each other to form discontinuous conductor portions.
It is accordingly one object of the present invention to provide a composition of metal powders which may be applied in dry form, or as an ink or paste containing binders, to a substrate, and which has minimal shrinkage and distortion upon being cured, by melting, into solid electrical conductors.
Another object is to provide a new and improved metal particle composition for additive printing of printed circuit boards.
Still another object is to control the metallurgical flow of a printed mass of metal powder when heated to a partial melt by using a specific particle size distribution.
Yet another object is to provide a metal particle composition particularly suited to curing by rapid electromagnetic heating.
Yet another object is to provide a composition wherein the particle sizes are selected to minimize shrinkage while maintaining good edge resolution of the printed pattern conductor lines.